JPH0963586A - Organic electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic electrolyte secondary battery with good load characteristics at low temperature without remarkable drop in charging/ discharging capacity at room temperature. SOLUTION: In an organic electrolyte secondary battery using a lithium- containing transition metal chalcogenide as a positive active material, a mixture of graphitized carbon having a mean particle size of 9μm or more and low crystalline carbon having a mean particle size of 0.5-2μm is used as a negative active material. As the graphitized carbon, carbon having a spacing d002 of 3.36Å or less and a crystallite size in the diection of (c) axis Lc of 400Å or more is preferable. As the low crystalline carbon, carbon having a spacing d002 of 3.37Å or more and a crystalline size, in the direction of (c) axis Lc of 250Å or less is preferable, and the mixing ratio of the graphitized carbon and the low crystalline carbon is 98:2 to 85:15 by weight.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electrolyte secondary battery, and more particularly to improvement of carbon as a negative electrode active material thereof.

[0002]

2. Description of the Related Art Carbon is generally used as a negative electrode active material in an organic electrolyte secondary battery using a lithium-containing transition metal chalcogenide as a positive electrode active material. This carbon has a layered structure and is charged by intercalating lithium between the layers. This layered structure can be controlled by heat-treating the raw material. For example, when the heat treatment temperature is high in the carbon production process, the interlayer distance d ₀₀₂ becomes narrow, and in the existing natural graphite having the highest crystallinity, the interlayer distance d _002. Is 3.35Å
(0.335 nm), the crystallite size Lc in the c-axis direction becomes large, reaching 1000 Å or more. Further, when the raw material contains a relatively large number of benzene rings and, for example, the raw material having a large number of carbon primary bonds, the interlayer distance d ₀₀₂ and the crystallite size Lc in the c-axis direction are different even at the same heat treatment temperature. It will be.

By the way, in an organic electrolyte secondary battery in which these carbons are used as a negative electrode active material, for example, a lithium-containing transition metal chalcogenide such as LiNiO ₂ or LiCoO ₂ is used as a positive electrode active material, graphitized carbon is used. Higher charge / discharge capacity can be obtained when used as the negative electrode active material, and theoretically C ₆ Li, that is, 372 m
A charge / discharge capacity of Ah / g is to be obtained.

However, as the graphitization progresses, the interlayer distance d ₀₀₂ becomes smaller as described above, so that the diffusion rate in the bulk (solid) of Li (lithium) ions which are mobility (mobile ions) in this secondary battery. Is reduced,
As a result, high current density (for example, about ² mA / cm ² )
Charge and discharge capacity tends to decrease. That is, the load characteristics of the battery tend to deteriorate. Since this is due to the physical properties inherent in the bulk, there is no improvement method other than reducing the graphitization degree of carbon.Therefore, in a battery using carbon having such a defect as a negative electrode active material, constant current-constant Since the charging method called the voltage method is adopted and the charging current density is attenuated in the latter half of charging, practically no problems have occurred so far.

However, in order for the present inventor to study the charging characteristics under various conditions in which the battery may be charged, the charge and discharge capacity was measured by changing the temperature and current density during charging of the battery. However, it was found that at low temperatures (0 ° C. or lower), the charge / discharge capacity was extremely reduced, and as a result of a competitive reaction with intercalation of lithium into carbon, lithium was very likely to be deposited on the negative electrode surface. That is, when the potential is not uniform in each part of the negative electrode, lithium is likely to be deposited at a low potential, which causes a problem of short circuit and ignition of the battery.

[0006]

DISCLOSURE OF THE INVENTION The present invention solves the problems of the conventional organic electrolyte secondary battery as described above, has good load characteristics, especially low temperature load characteristics, and has a negative electrode surface. An object is to provide a highly safe organic electrolyte secondary battery in which lithium is less likely to deposit.

[0007]

The present invention is directed to an organic electrolyte secondary battery having a lithium-containing transition metal chalcogenide as a positive electrode active material, and a graphitized carbon having an average particle diameter of 9 μm or more and an average particle diameter of 0 as an anode active material. The above object was achieved by using a mixture with low crystalline carbon of 0.5 to 2 μm.

That is, in the present invention, the voids between the particles of the graphitized carbon having a large charge / discharge capacity at room temperature are filled with low crystalline carbon having a specific particle diameter of 0.5 to 2 μm. As a result, a low-temperature load characteristic is improved, lithium is prevented from depositing on the negative electrode surface, a low-temperature load characteristic is good, and a highly safe organic electrolyte secondary battery in which lithium is less likely to deposit on the negative electrode surface is obtained. It is thought that it has become like this.

The process of the present invention will be described in detail as follows.

In order to improve low temperature load characteristics, a method of improving an electrolytic solution is generally used. However, the present inventor has found that the low crystalline carbon significantly improves low temperature load characteristics as a result of measuring the charge / discharge capacity by performing a charge / discharge test in which the above temperature and current density are changed, and this low crystalline carbon is found. It was made possible to provide an organic electrolytic solution secondary battery having good low temperature load characteristics without significantly reducing the charge / discharge capacity at room temperature by mixing with the graphitized carbon.

Low crystalline carbon generally has a property opposite to that of highly crystallized graphitized carbon, has a larger interlayer distance d ₀₀₂ than graphitized carbon, and is less likely to deposit lithium, but has a high charge / discharge capacity. There is a problem of being low. Therefore, when low-crystalline carbon having the same particle size as that of graphitized carbon is mixed, the charge / discharge capacity decreases depending on the weight ratio, but in order to improve low temperature load characteristics, the weight ratio of low-crystalline carbon should be increased. There is a circumstance that it must be.

On the other hand, when carbon black or acetylene black which is generally used as a conductive auxiliary agent is used, the low temperature load characteristics can be improved with a small amount of 10% by weight or less of the low crystalline carbon, but the volume is small. The ratio is large, and because its specific surface area is too large,
The ratio of the difference between the charge capacity and the discharge capacity (irreversible charge capacity) generated in the first cycle to the discharge capacity (hereinafter,
There is a problem that "retention" becomes large.

Therefore, the present inventor has studied a method of reducing both the weight ratio and the volume ratio of the low crystalline carbon to be added, and as a result, the low crystalline carbon having a specific particle diameter of 0.5 to 2 μm is used. Therefore, it has been found that the low temperature load characteristics can be improved without causing a significant decrease in charge / discharge capacity and an increase in retention.

That is, when the average particle size of the low crystalline carbon is smaller than 0.5 μm, the specific surface area is large, so that the retention is large, and when the average particle size of the low crystalline carbon is larger than 2 μm, it is per volume of the negative electrode. Discharge capacity decreases.

On the other hand, graphitized carbon has an average particle size of 9 μm.
Use the thing of m or more. This is because if the particle size of the graphitized carbon becomes too small, the charge / discharge capacity at room temperature may decrease, so it is necessary to use particles having an average particle size of 9 μm or more. . However, if the particle size of the graphitized carbon becomes too large, the volume density of the negative electrode may decrease, and the battery capacity may decrease. Therefore, the graphitized carbon having an average particle size of 9 to 20 μm is most suitable.

Further, in addition to the improvement of the low temperature load characteristics as described above, addition of low crystalline carbon to graphitized carbon increases micropores, thereby improving the liquid absorbing property of the electrolytic solution, and By increasing the expansion rate,
The strength of the negative electrode in the battery is improved, and as a result, the charge / discharge cycle is also increased.

The mixing ratio of the graphitized carbon and the low crystalline carbon is not particularly limited, but the weight ratio is 98 :.
2 to 85:15 are preferable, and particularly 95: 5 to 87:13.
Is preferred. If the amount of graphitized carbon is more than the above range, it may not be possible to sufficiently improve the low temperature load characteristics due to the reduction of low crystalline carbon, and if the amount of graphitized carbon is less than the above range, room temperature In this case, the charge / discharge capacity may decrease and the retention may increase.

In the present invention, examples of the positive electrode active material include lithium nickel oxide, lithium manganese oxide and lithium cobalt oxide (these are usually represented by LiNiO ₂ , LiMnO ₂ and LiCoO ₂ , respectively. Ratio of Li to Ni, ratio of Li to Mn, L
The ratio of i to Co is often deviated from the stoichiometric composition), and a lithium-containing transition metal chalcogenide is used alone or as a mixture of two or more kinds. However,
The positive electrode active material exists as the above compound when the battery is in a discharged state, and when the battery is in a charged state, it exists in a state in which lithium is released.

In the positive electrode, a conductive auxiliary agent such as phosphorus (scale) graphite, acetylene black or carbon black is added to the above positive electrode active material, if necessary, and, for example, polyvinylidene fluoride, tetrafluoroethylene or ethylene propylene. A positive electrode mixture prepared by adding a binder such as a diene terpolymer is pressure-molded, or a solvent is further added to form a paste, which is made of metal foil (for example, aluminum foil, titanium foil, platinum foil, etc.) It is manufactured through a process of coating and drying on a current collector. However, the method for producing the positive electrode is not limited to the above-described example.

The negative electrode has a negative electrode active material composed of the above-mentioned specific carbon mixture (however, the negative electrode active material exists as carbon itself when the battery is in a discharged state,
When the battery is in a charged state, lithium ions are intercalated between the layers), if necessary, for example, polyvinylidene fluoride or polytetrafluoroethylene, a binder such as ethylene propylene diene terpolymer is appropriately added, The negative electrode mixture prepared by mixing is pressure-molded, or a solvent is further added to form a paste, and the paste is applied onto a current collector made of metal foil (for example, copper foil, nickel foil, etc.), It is produced through a drying process. However, the method for producing the negative electrode is not limited to the above-exemplified method.

Examples of the organic electrolytic solution (hereinafter, simply referred to as "electrolytic solution" unless a battery is shown) include 1,2-dimethoxyethane, 1,2-diethoxyethane, propylene carbonate, ethylene carbonate,
γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, diethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, etc. alone or 2
In a mixed solvent of one or more kinds, for example, LiCF ₃ SO ₃ , Li
C ₄ F ₉ SO ₃ , LiClO ₄ , LiPF ₆ , LiBF
Electrolyte such as ₄ is used alone or in which two or more kinds are dissolved.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the present invention will be described more specifically with reference to Examples. However, the present invention is not limited to only those examples.

Example 1 As graphitized carbon, bulk carbon obtained by heat treatment of coke at 3000 ° C. was crushed to obtain an average particle size of 10 μm.
m powder was prepared. Interlayer distance d of this graphitized carbon
₀₀₂ is 3.36Å, and the crystallite size Lc in the c-axis direction
Was 853Å. Further, as low crystalline carbon, a product obtained by heat-treating coke at 800 ° C. was pulverized to prepare a powder having an average particle size of 1.5 μm. The interlayer distance d ₀₀₂ of this low crystalline carbon was 3.79Å, and the crystallite size Lc in the c-axis direction was 210Å.

These graphitized carbon and low crystalline carbon were mixed at a weight ratio of 95: 5, and 100 parts by weight of the mixture of the graphitized carbon and low crystalline carbon was mixed with 10 parts by weight of polyvinylidene fluoride as a binder. Then, 80 parts by weight of N-methyl-2-pyrrolidone was mixed as a solvent to prepare a carbon paste. The carbon paste was applied onto a copper foil as a current collector, dried and then pressed with a calendar roll to prepare a carbon electrode used as a negative electrode. In the preparation of the above carbon paste, polyvinylidene fluoride was previously dissolved in N-methyl-2-pyrrolidone.

On the other hand, the positive electrode was manufactured as follows. That is, as the positive electrode active material, lithium nickel oxide (usually expressed as LiNiO ₂ , but the ratio of Li and Ni is slightly deviated from the stoichiometric composition) was used, and this lithium nickel oxide and phosphorous graphite were used. An active material-containing paste for forming a positive electrode containing polyvinylidene fluoride in the following ratio was prepared. Lithium nickel oxide 91 parts by weight Phosphorous graphite 6 parts by weight Polyvinylidene fluoride 3 parts by weight

The preparation of the above-mentioned active material-containing paste for forming the positive electrode was carried out by dissolving polyvinylidene fluoride in N-methyl-2-methylpyrrolidone in advance, and adding lithium nickel oxide and phosphorous graphite thereto and mixing them. This was done by adding N-methylpyrrolidone and mixing.

The obtained active material-containing paste for forming a positive electrode was applied onto an aluminum foil having a thickness of 20 μm using an applicator, dried and then pressed with a calendar roll to produce a positive electrode.

As the electrolytic solution, a solution obtained by dissolving 1 mol / liter of LiPF _{6 in} a mixed solvent of ethylene carbonate and 1,2-dimethoxyethane in a volume ratio of 1: 1 was used. , Outer diameter 20mm, height 5m
A m-type button-type organic electrolyte secondary battery was produced.

In FIG. 1, 1 is the above positive electrode, and 2
Is the above negative electrode. Reference numeral 3 denotes a separator made of a microporous polypropylene film, and reference numeral 4 denotes an electrolyte absorber made of a polypropylene nonwoven fabric. 5 is a stainless steel positive electrode can, 6 is a positive electrode current collector made of aluminum foil,
Reference numeral 7 is a negative electrode can made of stainless steel and having a surface plated with nickel.

Reference numeral 8 is a negative electrode current collector made of copper foil, and 9 is a polypropylene annular gasket. In this battery, LiPF ₆ was used in a mixed solvent of ethylene carbonate and methyl ethyl carbonate in a volume ratio of 1: 1. An electrolyte solution in which 1 mol / liter of is dissolved is injected.

Example 2 As graphitized carbon, coke 3 was used in the same manner as in Example 1.
Bulk carbon obtained by heat treatment at 000 ° C. was crushed to prepare a powder having an average particle size of 10 μm. The interlayer distance d ₀₀₂ of this graphitized carbon was 3.36Å, and the crystallite size Lc in the c-axis direction was 853Å. Further, as low crystalline carbon, a product obtained by heat-treating coke at 800 ° C. was pulverized to prepare a powder having an average particle size of 0.5 μm. The interlayer distance d ₀₀₂ of this low crystalline carbon is 3.79Å
And the crystallite size Lc in the c-axis direction was 210Å.

The graphitized carbon and the low crystalline carbon were mixed at a weight ratio of 95: 5, and 100 parts by weight of the mixture of the graphitized carbon and the low crystalline carbon was mixed with 10 parts by weight of polyvinylidene fluoride as a binder. Then, 80 parts by weight of N-methyl-2-pyrrolidone was mixed as a solvent to prepare a carbon paste. The carbon paste was applied onto a copper foil as a current collector, dried and then pressed with a calendar roll to produce a carbon electrode.
Example 1 except that this carbon electrode was used as the negative electrode
A button type organic electrolyte secondary battery was produced in the same manner as in.

Example 3 As the graphitized carbon, coke 3 was used in the same manner as in Example 1.
Bulk carbon obtained by heat treatment at 000 ° C. was crushed to prepare a powder having an average particle size of 10 μm. The interlayer distance d ₀₀₂ of this graphitized carbon was 3.36Å, and the crystallite size Lc in the c-axis direction was 853Å. Further, as low crystalline carbon, furan resin obtained by heat treatment at 1100 ° C. was pulverized to prepare a powder having an average particle size of 1.5 μm. The interlayer distance d ₀₀₂ of this low crystalline carbon is 3.76.
And the crystallite size Lc in the c-axis direction was 41Å.

The graphitized carbon and the low crystalline carbon were mixed at a weight ratio of 95: 5, and 100 parts by weight of the mixture of the graphitized carbon and the low crystalline carbon was mixed with 10 parts by weight of polyvinylidene fluoride as a binder. Then, 80 parts by weight of N-methyl-2-pyrrolidone was mixed as a solvent to prepare a carbon paste. The carbon paste was applied onto a copper foil as a current collector, dried and then pressed with a calendar roll to produce a carbon electrode.
Example 1 except that this carbon electrode was used as the negative electrode
A button type organic electrolyte secondary battery was produced in the same manner as in.

Example 4 As graphitized carbon, bulk carbon obtained by heat treatment of coke at 3000 ° C. was crushed to obtain an average particle size of 20 μm.
m powder was prepared. Interlayer distance d of this graphitized carbon
₀₀₂ is 3.36Å, and the crystallite size Lc in the c-axis direction
Was 790Å. Further, as low crystalline carbon, a product obtained by heat-treating coke at 800 ° C. was pulverized to prepare a powder having an average particle size of 2.0 μm. The interlayer distance d ₀₀₂ of this low crystalline carbon was 3.80Å, and the crystallite size Lc in the c-axis direction was 25Å.

These graphitized carbon and low crystalline carbon were mixed at a weight ratio of 87:13, and 100 parts by weight of the mixture of the graphitized carbon and low crystalline carbon was mixed with 10 parts by weight of polyvinylidene fluoride as a binder. Then, 80 parts by weight of N-methyl-2-pyrrolidone was mixed as a solvent to prepare a carbon paste. The carbon paste was applied onto a copper foil as a current collector, dried and then pressed with a calendar roll to produce a carbon electrode. A button type organic electrolyte secondary battery was produced in the same manner as in Example 1 except that this carbon electrode was used as the negative electrode.

Comparative Example 1 A button type organic electrolyte secondary battery was prepared in the same manner as in Example 1 except that low crystal carbon was not used and the amount of graphitized carbon was increased accordingly.

Comparative Example 2 Button-shaped organic electrolyte solution 2 was prepared in the same manner as in Example 1 except that low crystalline carbon obtained by heat treatment of coke at 800 ° C. was used as powder having an average particle size of 12 μm. A secondary battery was produced.

Comparative Example 3 A button type organic electrolyte secondary battery was prepared in the same manner as in Example 1 except that acetylene black having an average particle size of 43 mμm was used as the low crystalline carbon.

The batteries of Examples 1 to 4 and Comparative Examples 1 to 3 produced as described above were stored at 20 ° C. at a charge / discharge current density of 0.
5 mA / cm ^2, discharge capacity when were charged and discharged in the range of voltage 10MV～1V, retention, 0 ° C., the charge and discharge current density of 2 mA / cm ^2, voltage -0.03~1V (vs Li
/ Li ⁺ ), the discharge capacity when charged and discharged, the electrolyte absorption rate (the difference in the weight of the negative electrode before and after immersion in the electrolytic solution, divided by the weight of the negative electrode excluding the weight of the copper foil as the current collector), and The number of cycles until the discharge capacity was reduced to 80% of the initial value when charging / discharging was performed at a charge / discharge current density of 0.5 mA / cm ² and a voltage of 10 mV to 1 V was examined. The results are shown in Tables 1 and 2.

[0041]

[Table 1]

[0042]

[Table 2]

As is clear from the comparison between the characteristics of Examples 1 to 4 shown in Table 1 and the characteristics of Comparative Examples 1 to 3 shown in Table 2, Examples 1 to 4 have a large discharge capacity at 0 ° C. It has good low temperature load characteristics, many cycles, and room temperature (2
The discharge capacity at 0) ° C. was large, the high charge / discharge capacity was maintained, and the increase in retention was small.

On the other hand, in Comparative Example 1, since only graphitized carbon having high crystallinity is used, the charge / discharge capacity at room temperature is large, but the discharge capacity at 0 ° C. is small and the low temperature load characteristics are low. It was bad and the number of cycles was small. In Comparative Example 2, the average grain size of the low crystalline carbon was as large as 12 μm, so that the charge / discharge capacity at room temperature was decreased and the number of cycles was small. Was too small, so retention was increased and the number of cycles was also small.

[0045]

As described above, in the present invention, by using the mixture of graphitized carbon and low crystalline carbon as the negative electrode active material, the charge and discharge capacity at room temperature is not significantly lowered and It was possible to provide an organic electrolyte secondary battery having excellent load characteristics.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of an organic electrolyte secondary battery according to the present invention.

[Explanation of symbols]

 1 Positive electrode 2 Negative electrode 3 Separator 4 Electrolyte absorber

Claims (4)

[Claims] 1. In an organic electrolyte secondary battery having a positive electrode using lithium-containing transition metal chalcogenide as a positive electrode active material, a negative electrode, and an organic electrolytic solution, the negative electrode active material has an average particle size of 9
An organic electrolyte secondary battery comprising a mixture of graphitized carbon having a size of at least μm and low crystalline carbon having an average particle size of 0.5 to 2 μm. 2. The organic electrolyte secondary battery according to claim 1, wherein the mixing ratio of the graphitized carbon and the low crystalline carbon is 98: 2 to 85:15 by weight. 3. The interlayer distance d ₀₀₂ of graphitized carbon is 3.
Below 36Å, the crystallite size Lc in the c-axis direction is 400Å
It is above, The organic electrolyte secondary battery of Claim 1 or 2 characterized by the above-mentioned. 4. The interlayer distance d ₀₀₂ of low crystalline carbon is 3.
Crystallite size Lc in the c-axis direction is 250Å above 37Å
The organic electrolyte secondary battery according to claim 1 or 2, wherein: